The invention relates to a lithography projection objective for imaging a pattern arranged in an object plane of the projection objective onto a substrate to be arranged in an image plane of the projection objective.
The invention further relates to a method for correcting image defects in the case of a lithography projection objective that can be tuned to immersion operation.
A projection objective of the type mentioned at the beginning is preferably used for microlithography projection exposure machines for producing semiconductor components and other finely structured subassemblies. A projection objective serves the purpose of projecting patterns from photomasks or graticules, which are also designated as masks or reticles, onto an object, that is to say a substrate, coated with a photosensitive layer, or onto a semiconductor wafer coated with photoresist, with very high resolution.
The resolution of the imaging of the pattern by the projection objective is proportional to the wavelength of the light used, and inversely proportional to the image-side numerical aperture of the projection objective. The resolution can therefore be improved with the aid of shorter wavelengths and higher numerical apertures. The numerical aperture NA is given by NA=n·sin Θ, n being the refractive index of the medium between the substrate and the last optical element of the projection objective.
Hitherto, use has predominantly been made of projection objectives in the case of which there exists in the image space between the exit surface of the last optical element of the projection objective and the image plane a finite working distance that is filled during operation with air or another suitable gas. Such systems are designated as “dry systems” or “dry objectives”. The working distance between the last optical element and the substrate is generally filled in this case with helium, nitrogen or another gas or a gas mixture with a refractive index n of approximately 1.
It follows from the previously mentioned relationship between the resolution and the image-side numerical aperture that the resolution can be raised when an immersion medium of high refractive index is introduced into the working distance between the exit surface of the last optical element and the substrate. This technique is designated as immersion lithography. A projection objective of this type is also designated as an “immersion system” or “immersion objective”. Some refractive projection objectives that are suitable for immersion lithography and have image-side numerical apertures NA>1 are disclosed in the patent applications DE 102 10 899 and PCT/EP 02/04846 of the same applicant.
A further advantage of an immersion objective consists in the possibility of obtaining a larger depth of field of the imaging in conjunction with the same numerical aperture as for a dry objective. This advantage is used in the projection objectives according to the invention.
In the case of an immersion objective, instead of being filled with a gas, the space between the exit surface of the last optical element of the projection objective and the substrate, which determines the working distance, is filled with an immersion medium of a refractive index substantially greater than 1. An immersion medium normally used at present is water, but it is possible, particularly within the scope of the present invention, to select other immersion media in accordance with needs and suitability.
Document EP 1 431 826 A2, which stems from the same applicant, describes how simple design means and manipulations can be used to tune a projection objective between a dry operation (dry configuration) and an immersion operation (immersion configuration). The projection objective described there has a multiplicity of optical elements that are arranged along an optical axis of the projection objective, the optical elements comprising a first group, following the object plane, of optical elements and a last optical element that follows the first group, is next to the image plane and defines an exit surface of the projection objective that is arranged at a working distance from the image plane. The last optical element is substantially free from refractive power and has no sag or only a slight one. The tuning method described there comprises varying the thickness of the last optical element, changing the refractive index of the space between the exit surface of the last optical element and the substrate by introducing or removing an immersion medium, and moreover preferably an axial displacement of the last optical element for the purpose of setting a suitable working distance in the dry operation of the projection objective. Moreover, it is provided to refine the tuning to the dry configuration or the immersion configuration by changing the air spaces between individual optical elements of the first group or by providing or varying aspheres.
The projection objective of the present invention can likewise be tuned between a dry configuration and an immersion configuration.
However, the present invention is based on a further aspect of such a projection objective that can be tuned between the dry configuration and the immersion configuration.
A temperature change usually occurs during operation of a projection objective. This can be global, homogenous or else local. For example, the air around the projection objective, the projection objective housing, the individual mounts of the optical elements, the optical elements themselves and the air or the gas inside the projection objective and, during immersion operation, the immersion liquid can heat up.
It has emerged that temperature changes have a different effect with regard to spherical image defects on a projection objective during immersion operation than on a projection objective in the dry configuration. In other words, dry objectives and immersion objectives differ from one another with regard to their sensitivity to temperature changes.
In the case of a projection objective in dry configuration, such spherical aberrations induced by temperature changes can be at least largely compensated even in the relatively high order by simply refocusing in which only the position of the substrate is adjusted in the direction of the optical axis. Specifically, a change in the working distance between the exit surface of the last optical element and the substrate leads in the case of a projection objective in dry configuration to very similar aberrations such as heating up of the projection objective, and so the aberrations induced by the heating up can be at least largely compensated by an appropriately directed displacement of the substrate, usually in conjunction with heating up, in a direction away from the last optical element.
It came out that this mode of procedure, specifically a correction of image defects on the basis of temperature changes solely by adjusting the position of the substrate does not lead in the case of a projection objective in immersion configuration to the result as in the case of a projection objective in dry configuration, that is to say in the case of such a focusing correction in which the Zernike coefficient Z4 is compensated to zero, higher spherical Zernike coefficients Z9, Z16, Z25, . . . remain and impair the imaging properties of the projection objective in immersion configuration.